Commercially polymers produced by solvent and bulk polymerization, and by condensation generate a polymerized, viscous reaction mass as the concentration of their liquid contaminants decreases. To remove these contaminants from this reaction mass requires a high level of heat and shear energy (stirring, mixing, agitation, turbulence) under exerted pressure, whether reduced/negative (that is, under less than 1 psia, preferably a vacuum) or positive (that is, elevated pressure optionally in the presence of an inert gas) atmosphere). It is immediately evident that increasing pressure is directly opposed to a goal of obtaining a high diffusion coefficient “D” for a contaminant to be removed from a viscous reaction mass; and the limit to which temperature may be increased is that at which the desired polymer begins either to degrade physically, evinced by a change of color, or to suffer chain scission, or both. Therefore, when the polymer is to contain additives such as colorants, light stabilizers, anti-oxidants, heat stabilizers and finely divided inert particulate fillers, these are typically added after the desired polymer has been purified. However, they may be added prior to decontamination, and a filler such as a nanoclay may facilitate pretreatment of a slurry or reaction mass whether by phase separation in the liquid phase, or by devolatilization of volatiles, and eventual decontamination of the polymer.
The term “decontamination” is used herein to describe removal of unwanted contaminants, whatever conventional unit operation may be used to remove them. A typical pretreatment step refers to removal of an unwanted component in a reaction mass either (a) during a polymerization reaction and prior to being further treated, or, (b) after a polymerization reaction has reached equilibrium, and prior to being further treated.
The term “reaction mass” connotes the output of a polymerization reactor, which output is to be treated in the process described herein. Such output may be obtained either (i) directly from the polymerization reactor, or (ii) from a pretreatment step in which the reaction mass is pretreated to remove as much of unwanted contaminants as may be necessary to reduce their concentration to no more than 20%. Thus, in a melt polymerization, the reaction mass may be taken directly from the reactor. In an interfacial polycondensation using water and organic phases, the slurry from the reactor includes the polymer, reaction byproducts, unreacted monomer, finely divided solids whether filler, processing additives or the like which may be mixed into the slurry, and small amounts of initiator, catalyst, chain terminating agent, chain transfer agent, and the like which may be used in the reaction, all of which are distributed between the organic and aqueous phases. When the organic/aqueous solvents are present in an amount in excess of 50% by weight (50% by wt) of the slurry, it is pretreated to remove a major portion of both the aqueous phase and the organic solvent phase and leave less than 20% by wt of contaminants. Pretreatment may include a single unit operation, or plural operations in combination.
There is an increasing demand for an essentially pure high Mw polymer having a molecular weight Mw above the critical molecular weight for entanglement Mc of the polymer, typically Mw greater than about 5,000, containing only a specified minimal concentration of unwanted contaminants. This is true whether the contaminants in the polymerized reaction mass are present in the range from 0.1% to 3% of the reaction mass, as they typically are, after decontamination to remove contaminants, but before additional purification; or, are present in a larger amount in the range from 5% to 20%, as they might be, if taken directly from a reactor for a melt polycondensation reaction, or even in a major proportion by weight, as they might be if taken directly from a reactior for a solution condensation or interfacial polycondensation, thereafter to be treated and decontaminated.
In such condensation reactions, the Mw of the polymer is limited not only by the formation and presence of by-products but also by the fact that the high concentration of tightly entangled long polymer chains have chain ends which cannot be accessed by remaining monomer. Therefore, if the byproducts are removed while the reaction is still in progress, remaining monomer molecules can “find” the reactive chain ends and increase the molecular weight.
Irrespective of the high Mw polymer, the degree of difficulty of devolatilizing the last 1000 ppm of contaminant is determined by the structure and morphology of the polymer, the rheology of its melt, and the degree of difficulty increases as the Mw of the polymer and the viscosity of the melt increase. This difficulty is typically most evident in polymers having a Mw in the range from about 10,000 to 40,000, and higher, depending upon the particular polymer and the rheological properties of a reaction mass which is to be decontaminated.
The Problem:
Removal of unwanted contaminants from a reaction mass becomes increasingly difficult in high Mw polymers, that is, those having Mw depending upon the polymer being made. It is desired to decontaminate then purify the reaction mass in which the desired polymer has a Mw>about 5,000, typically at the upper limit currently deemed commercially practical. Such polymers can presently be decontaminated to contain less than about 1000 ppm of volatiles, but to decontaminate them below 1000 ppm, preferably below 100 ppm, is an extreme problem. Further it is desired to make a higher Mw polymer than is currently practical, at a temperature and pressure lower than those required to make the same high Mw polymer using the same conventional process, using the same reactor and identical reactants.
DE-4,236,039 discloses a method for the production of condensation polymers requiring plural stages, each with a reactor and water removal equipment. The reaction mixture is circulated through tubular heat exchangers to remove a majority of the water by-product, driving the reaction to a higher degree of completion in a first stage. Additional byproduct water is removed in a secondary stage.
Relatively recently, to avoid high temperature and pressure, a centrifugal pelletizing process using a centrifugal devolatilizing apparatus described in U.S. Pat. No. 5,453,158 side-steps the problems associated with conventional low pressure, high temperature devolatilization. Yet another way to do so is to use a solid state polymerization process. However, like all the older conventional processes, neither one of these processes suggests modifying the state of entanglement of the polymer molecules, for any reason. Since in the diffusion equation controlling the mass of material diffusing through unit area in unit time, the thickness of the element of area across which diffusion occurs does not suggest the physical state of the molecules in that area, there was no reason to conclude that disentangling molecules in a polymerization reaction mass was likely greatly to increase “D” and correspondingly greatly increase the mass of material transferred under the same conditions which would prevent that transfer if the molecules were not disentangled.
Most commonly, devolatilization is carried out in a single or twin-screw vented devolatilizing extruder. However, vents are a particular problem in high-speed processing with materials that do not adhere well to barrel walls, such as very high molecular weight polyethylene or very highly filled materials. Further, a vented extruder is only useful where the contaminant to be removed is in the vapor phase.
It will be evident from the foregoing that when the desired polymer has Mw <5,000, the viscosity of the polymer is generally low enough that devolatilization is not a problem even with a thermally sensitive polymer. By “thermally sensitive” is meant that the additive suffers noticeable and/or unacceptable degradation or change in physical characteristics at the temperature of melt in which the additive is dispersed, such degradation being referred to as “thermal degradation”. When the thermally sensitive polymer has Mw>5,000 but less than 10,000, and the viscosity of the polymer is high enough that conventional devolatilization requires a temperature close to that at which the polymer is thermally degradable despite reduced pressure, the risk of devolatilization to a level below about 500 ppm is generally high. Most importantly, when the desired polymer has Mw>about 10,000, the viscosity of the polymer is often so high that effective devolatilization to below 500 ppm involves even higher risk becomes more difficult, and as the Mw increases above about 40,000, known processes and equipment for effective devolatilization to below 500 ppm become ineffective.
U.S. Pat. Nos. 5,885,495 and 6,210,030 issued to Ibar teach how to modify the physical and physico-chemical characteristics of a polymer melt, and how to make a “stress-fatigued” melt which is fluidizable at a temperature below the virgin polymer's conventional fluidization temperature. In the '495 process, virgin polymer, that is, conventionally manufactured solid polymer purchased in the market place, is extruded to form a melt which is then led into an apparatus referred to as a “TekFlow® processor”, or “processor” for brevity, available from Stratek Plastic Ltd. (Dublin, Ireland) and SPRL Inc. (Wallingford, Conn., USA).
In the '495 process viscosity is reduced by heating a polymer above its fluidization temperature to form a melt; submitting the melt, at constant temperature, to the action of a vigorous mechanical vibration, at a constant amplitude and at a frequency of up to 100 Hz, for a chosen time at that temperature, causing the melt to become highly elastic, and simultaneously causing it to fatigue; and adjusting the vibration parameters to maintain the high elastic state, until the state of entanglement between the macromolecules has altered to a desired level, as measured by a change in viscosity and melt modules of elasticity of the melt.
In this process, the melt is mechanically vibrated and fatigued until the state of entanglement between the molecules has been modified to a desired level of disentanglement as measured by a decrease of at least 5% in the viscosity and melt modulus of elasticity relative to that of the virgin melt after correction of the influence of degradation of the chains, on viscosity. The resulting polymer, referred to herein as being “disentangled”, “extensively shear-thinned”, or “stress-fatigued” is referred to herein as “modified” polymer (for brevity), and is characterized by having a fluidization temperature at least 10° C. lower than the fluidization temperature of the same virgin polymer had it not been extensively shear-thinned and stress-fatigued.
Because a reaction mass to be treated is already fluid, if not a liquid, “fluidization temperature” of a polymerized reaction mass is defined as that temperature at which the reaction mass leaves the reactor. This fluidization temperature is conveniently in the range from about 10° C. to 200° C. above (i) the measured melt temperature (at ambient temperature of 25° C. and atmospheric pressure) for the polymer made, if it is recovered in substantially crystalline form, or, (ii) the glass transition temperature Tg of an amorphous polymer, at which the polymer begins to flow, if the polymer is recovered as an amorphous polymer.
There is no suggestion in the '495 patent that a reaction mass having a polymer dissolved therein, would be as susceptible to the energy imparted by the '030 apparatus as a solvent-free melt. Theoretical considerations on rheology of concentrated solutions in the linear range (see J. D. Ferry in “Viscoelastic Properties of Polymers”, 2nd Edition, Wiley, Chapter 17 (1989)) do not help predict what will happen when a slurry is brought into the non-linear range of viscoelasticity, which produces disentanglements. There is no reason to expect that large polymer molecules diluted with solvent, and therefore relatively spaced-apart compared to polymer molecules in a melt, would be effectively disentangled; the higher the proportion of solvent, the more difficult one would expect it to be to “find” and disentangle the spaced-apart molecules.
The '030 apparatus is configured to provide internals specifically adapted for the reaction mass to be processed. Though the '030 processor does not show vents, such as may be used to remove volatiles while the reaction mass is being processed, the addition of vents may be effected using conventional technology analogous to that used for venting a screw extruder. The details of construction of the '030 apparatus are incorporated by reference thereto as if fully set forth herein.
Referring to a prior art apparatus which modifies the rheology of a melt of polymer molecules, the '495 patent also states: “The second category of patents and processes using vibration is based on the fact that material rheology is a function of vibration frequency and amplitude in addition to temperature and pressure. This can be put to practical use to influence diffusion and rate sensitive processes which depend on viscosity and relaxation kinetics, such as nucleation and growth of crystals, blending and orientation”. (col 2, lines 39-46). However, there is no suggestion as to how one might implement any desired influence on diffusion, or any modification of a rate sensitive process.
In particular, it is known that the diffusion coefficient of a molecule is a function of the viscosity of the liquid in which the molecule is held, the absolute temperature and the effective radius of the molecule. This relationship is given by the equation: D=kbT/6πηr where D=diffusion coefficient; kb=Boltzman's constant; T=absolute temperature; η=viscosity; and, r=effective radius of the molecule. Knowing that the '495 process is effective to disentangle large molecules, and to decrease ‘η’ of the polymer, but not knowing how disentanglement affects ‘r’, it is not possible to know how the interaction of η and r might affect the diffusion coefficient of a contaminant molecule. Nevertheless, since the ‘r’ of a contaminant molecule is typically small relative to the polymer molecules, one would expect that a change in ‘η’ of the reaction mass would change “D” of the contaminant molecule through the reaction mass, but how such change will affect the processability of a melt and facilitate the devolatilization of a contaminant from the melt cannot be logically deduced. (See “Chain Dynamics in Entangled Polymers: Diffusion versus Rheology and Their Comparison” by S. Q. Wang, J. Polym. Sci., part B, Vol. 41, 1589 (2003)).